Less Can Be More When Evolving Against Epidemics

March 30, 2012

When it comes to battling a deadly parasite epidemic, it may be better not to fight it. A new study of freshwater zooplankton suggests less is more when it comes to evolving against a parasitic epidemic.

The zooplankton, known as Daphnia dentifera, has been found to endure a yeast parasite infection known to affect more than 60% of its population. What scientists found interesting is how quickly the Daphnia population evolves, balancing infection, resistance, and reproduction.

According to the study, several factors are at play determining the size of the epidemic and how quickly these “water fleas” are able to evolve to survive these epidemics. For instance, the number of vertebrae predators in the water can affect the size of the epidemic as well as the available food source for the Daphnia.

The National Science Foundation and the James S. McDonnel Foundation provided funding for this study, which was published in this weeks issue of the journal Science.

In a NSF press release, Saran Twombly, program director of NSF´s Division of Environmental Biology said, “This study is a great example of why the most obvious response to disease, increased resistance, may not be the best solution.”

“When populations are stressed by other factors such as food or predators, remaining susceptible to a disease is the best route to long-term success.”
The study found lakes with higher concentrations of nutrients and lower levels of predators are more susceptible to larger epidemics. In these conditions, the yeast has less of an effect on Daphnia as they become more resistant to infection.

In an alternate environment, however, where food is scarce and predators are abundant, the epidemics remain small and Daphnia evolve increased susceptibility to the parasite.

Such findings have surprised scientists, as this seems to go against normal reactions.

“It´s counterintuitive to think that hosts would ever evolve greater susceptibility to virulent parasites during an epidemic, but we found that ecological factors determine whether it is better for them to evolve enhanced resistance or susceptibility to infection,” said the paper´s lead author Meghan Duffy, a biologist at Georgia Tech.

According to Duffy, the more resources spent of trying to evolve resistance, fewer resources are available for reproduction, and in these waters, it is better to reproduce than resist.

A sort of microbial “Make love not war,” if you will.

Researchers studied the levels of nutritional resources, predation, and parasitic infection in Indiana lakes weekly for a period of 4 months.
To sum up their results, Duffy told nsf.gov “Daphnia became more susceptible to the yeast in lakes with fewer resources and higher vertebrate predation, but evolved toward increased resistance in lakes with increased resources and lower predation.”

These results confirm the teams hypothesis: When a host faces a resistance-reproduction tradeoff, they will evolve a higher resistance during larger epidemics as opposed to smaller ones.

The team will continue their research this summer in Indiana lakes to study the relationship between ecological factors and epidemic size to corroborate their results.